1. Field of the Disclosure
The present disclosure relates to a voltage detection device that is mounted on a vehicle and detects a decrease in an output voltage of a power supply device of the vehicle.
2. Description of the Related Art
Recently key-less entry systems include an in-vehicle communication device that is mounted on a vehicle and a portable device capable of wireless communication with the in-vehicle communication device. With such systems, vehicle operations, such as locking and unlocking of vehicle doors, may be performed by using a wireless communication between' the in-vehicle communication device and the portable device without using a mechanical key. The key-less entry system often has an immobilizer function in which permission or non-permission for starting the engine with a mechanical key is determined by performing verification of ID information by using wireless communication between the in-vehicle communication device and the portable device, in addition to the key-less function in which vehicle operations, such as locking and unlocking of vehicle doors, are performed without using a mechanical key.
Furthermore, in recent years, in accordance with an increase in the number of in-vehicle electronic devices mounted on a vehicle, dissemination of vehicles having an idling stop function, and the like, a case where the remaining amount of a battery (power supply device) decreases frequently occurs. Some in-vehicle electronic devices including the in-vehicle communication device of the key-less entry system operate even while the engine is turned off. Therefore, if a vehicle is left unattended for a long period of time with the engine turned off when the remaining amount of the battery is decreasing, an output voltage of the battery will decrease. Moreover, when a cranking operation (an operation to rotate the engine with a motor for starting the engine) is performed to start the engine while the remaining amount of the battery is decreasing, high power is used to actuate the motor for starting the engine. Therefore, the output voltage of the battery may decrease even further. When the output voltage of the battery is decreased, a circuit of the in-vehicle communication device may not operate properly, and may generate a defect in the operation of the key-less entry system.
Therefore, there is an increase in demand for a key-less entry system capable of avoiding defective operation of the system even in the case where the remaining amount of the battery decreases and the output voltage of the battery largely decreases. As a measure to avoid such a defect, providing a voltage detection device that detects the output voltage of the battery and employing a method for temporarily stopping a function of the key-less entry system before a circuit of the in-vehicle communication device stops operating properly in the case where the output voltage of the battery decreases, is effective.
As a technique related to the voltage detection device, for example, a device such as a power diagnostic device for a vehicle according to Japanese Unexamined Patent Application Publication No. 2000-206215 has been proposed. FIG. 7 is an explanatory diagram illustrating a configuration of a power diagnostic device for a vehicle 201 according to Japanese Unexamined Patent Application Publication No. 2000-206215. As illustrated in FIG. 7, the power diagnostic device for a vehicle 201 includes current detection means 203, terminal voltage detection means 204 (voltage detection device), abnormality determination means 206, current limitation means 207, abnormality warning means 208, a battery 202 (power supply device), an ignition switch 209, a back-up power supply 210, and a power generator 211.
The current detection means 203 detects a current flowing through an electronic device that is connected to the battery 202. The terminal voltage detection means 204 detects a terminal voltage of the battery 202 while the ignition switch 209 is turned off. When the terminal voltage of the battery 202 is at or below a specific value, the abnormality determination means 206 determines that abnormality has occurred. When the abnormality determination means 206 determines that abnormality has occurred, the current limitation means 207 limits on the current flowing through the electronic device, and the abnormality warning means 208 issues a warning.
Japanese Unexamined Patent Application Publication No. 2000-206215 does not mention stopping of a function of the key-less entry system. However, by using the above-mentioned technique, it is possible to detect the output voltage of the battery and to stop a function of the key-less entry system at an appropriate timing, when the output voltage of the battery decreases.
Furthermore, Japanese Unexamined Patent Application Publication No. 2000-206215 does not disclose a particular circuit configuration of the terminal voltage detection means 204 (voltage detection device). However, for example, a device having a circuit configuration similar to a conventional voltage detection device 310 illustrated in FIG. 8 may be used as the terminal voltage detection means 204. FIG. 8 is an explanatory diagram illustrating the circuit configuration of the voltage detection device 310.
As illustrated in FIG. 8, the voltage detection device 310 includes a voltage-dividing circuit 311, a constant-voltage circuit 312, and a detection circuit 313. Furthermore, the voltage detection device 310 is connected to a battery 320 (power supply device) and a control device 330. The battery 320 is a device equivalent to the battery 202 of the power diagnostic device for a vehicle 201 according to
Japanese Unexamined Patent Application Publication No. 2000-206215. The control device 330 is a circuit equivalent to the abnormality determination means 206 of the power diagnostic device for a vehicle 201 according to Japanese Unexamined Patent Application Publication No. 2000-206215.
The voltage-dividing circuit 311 divides an output voltage VB of the battery 320 with a predetermined ratio. The voltage-dividing circuit 311 includes a resistive element R31 and a resistive element R32 that are connected in series in such a manner that the resistive element R31 and the resistive element R32 are in a predetermined voltage-dividing ratio. A connection point of the resistive element R31 and the resistive element R32 is an output terminal of the voltage-dividing circuit 311. A ground capacitance C31 is connected to the output terminal of the voltage-dividing circuit 311. The ground capacitance C31 is a capacitance to eliminate noise. An output voltage V1 of the voltage-dividing circuit 311 varies in proportion to the output voltage VB of the battery 320. The constant-voltage circuit 312 converts the output voltage VB of the battery 320 into a predetermined voltage V2. A circuit including a regulator IC or the like may be used as the constant-voltage circuit 312.
The output voltage V1 of the voltage-dividing circuit 311 and the output voltage V2 of the constant-voltage circuit 312 are input to the detection circuit 313. The output voltage V2 of the constant-voltage circuit 312 is used as a power-supply voltage Vcc and a reference voltage Vref of the detection circuit 313. Furthermore, based on the output voltage V1 of the voltage-dividing circuit 311 and the reference voltage Vref, the detection circuit 313 estimates the output voltage VB of the battery 320, and detects a decrease in the output voltage VB of the battery 320. Information regarding the decrease in the output voltage VB of the battery 320 is transmitted from the detection circuit 313 to the control device 330. Then, when the output voltage VB of the battery 320 decreases and reaches a specific value or less, the control device 330 determines that abnormality has occurred.
In the conventional voltage detection device 310, the decrease in the output voltage of the battery 320 is detectable if the decrease in the output voltage of the battery 320 is within a specific range. However, if the remaining amount of the battery 320 largely decreases and the output voltage of the battery 320 decreases to a value near the reference voltage Vref, the constant-voltage circuit 312 is not able to maintain a specific output voltage, thus outputting a voltage that is lower than the original reference voltage Vref.
As a result, the detection circuit 313 starts to estimate the output voltage of the battery 320 based on the voltage that is lower than the reference voltage Vref. Therefore, the decrease in the output voltage of the battery 320 cannot be detected correctly. Furthermore, for example, a case may occur in which the control device 330 determines that the output voltage of the battery 320 is higher than a specific value, even when the output voltage of the battery 320 has actually reached the specific value or less. In such a case, it is not possible to stop a function of the key-less entry system at an appropriate timing, when the output voltage of the battery 320 decreases, and therefore there is a possibility of generating a defect in an operation of the system.
The present invention has been made in view of the circumstances of such a related art, and provides a voltage detection device capable of detecting a decrease in an output voltage of a power supply device, even in the case where the output voltage of the power supply device largely decreases.